Starter will need to remain 6v, otherwise the starter will rotate too slow to start engine and will overheat. Most starters within a manufacturer (e.g. GM) have similar bolt patterns and drive gears and can usually be swapped.
Ignition is not a problem until you get to modern electronic systems.
Other engine areas to consider are instrument sensors (coolant temperature, oil pressure,) and actuators (electric choke). Most sensors can be replaced with 6v sensors, choke can be changed to manual.
A 6 volt starter can safely be run on 12 volts, as it only runs for a very short period. On 12 volts, a 6 volt starter will actually turn over faster, not slower. There is a risk of overheating, but only if the starter is cranked for an exessive period.
"Sensors" will not be an issue with stock gauges, coolant and oil gauges on the original gauges are mechanical, not electric. Only the gas gauge and lighting would be an issue with a 12 volt conversion. However, as mentioned, a 12 volt conversion is not neccessary for an engine swap.
Just out of curiosity, though, is there a reason you're not running the GMC engine? The GMC 6 is considered by many to be superior to the Chevrolet engine.
